1. Field of the Invention
The present invention relates to a display device such as organic EL (Electro Luminescence) display having pixel circuits arranged in a matrix form, each of which has an electro-optical element whose brightness is controlled by a current. The invention relates particularly to a so-called active matrix display device in which the current flowing through an electro-optical element is controlled by an insulated gate field effect transistor disposed in each pixel circuit.
2. Description of the Related Art
An image display device such as liquid crystal displays an image by controlling the optical intensity of each pixel according to image information to be displayed. This is also true for organic EL and other displays. However, organic EL display is a so-called spontaneous luminescent display having a light-emitting element in each pixel circuit. This type of display offers advantages including high image visibility, no need for backlights and high response speed.
Further, organic EL display differ significantly from liquid crystal display and other types of display in that the brightness of each light-emitting element is controlled by a current flowing therethrough to provide color gradation. That is, the light-emitting elements are current-controlled.
As with liquid crystal displays, organic EL displays can be driven by simple or active matrix. It should be noted, however, that although the formers are simple in structure, they have disadvantages including difficulties in implementing a large-size, high-definition display. As a result, the development of active matrix displays has been going on at a brisk pace in recent years. In this type of display, the current flowing through the electro-optical element in each pixel circuit is controlled by an active element, which is generally a TFT (Thin Film Transistor), provided in the same pixel circuit.
FIG. 1 is a block diagram illustrating the configuration of a typical organic EL display device.
A display device 1 includes a pixel array section 2 having pixel circuits (PXLCs) 2a arranged in an m by n matrix. The display device 1 further includes a horizontal selector (HSEL) 3, write scanner (WSCN) 4, data wirings DTL1 to DTLn and scan lines WSL1 to WSLm. The data wirings DTL1 to DTLn are selected by the horizontal selector 3 and supplied with a data signal commensurate with brightness information. The scan lines WSL1 to WSLm are selectively driven by the write scanner 4.
It should be noted that the horizontal selector 3 and write scanner 4 may be formed on a polycrystalline silicon or around pixels using, for example, a MOSIC.
FIG. 2 is a circuit diagram illustrating a configuration example of the pixel circuit 2a in FIG. 1 (refer to, for example, U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683).
The pixel circuit 2a in FIG. 2, which is the simplest in configuration of a large number of circuits proposed, is a so-called dual-transistor drive circuit.
The pixel circuit 2a in FIG. 2 includes p-channel thin film field effect transistors (hereinafter referred to as “TFTs”) 11 and 12, a capacitor C11, and an organic EL element (OLED) 13 which is a light-emitting element. In FIG. 2, DTL and WSL represent the data wiring and scan line, respectively.
An organic EL element has often rectifying capability. As a result, it is sometimes referred to as an OLED (Organic Light Emitting Diode). Although represented by a diode symbol in FIG. 2 and other drawings, the organic EL element is not necessarily demanded to offer rectifying capability in the description given below.
In FIG. 2, the TFT 11 has its source connected to a supply potential VCC. The light-emitting element 13 has its cathode connected to a ground potential GND. The pixel circuit 2a in FIG. 2 operates in the manner described below.
Step ST1:
The scan line WSL is placed into a selected state (pulled down to low level in this case). Then, a write potential Vdata is applied to the data wiring DTL. As a result, the TFT 12 conducts, charging or discharging the capacitor C11 and bringing the gate potential of the TFT 11 to Vdata.
Step ST2:
The scan line WSL is placed into an unselected state (pulled up to high level in this case). This causes the data wiring DTL and TFT 11 to be electrically isolated from each other. However, the gate potential of the TFT 11 is maintained constant by the capacitor C11.
Step ST3:
The current flowing through the TFT 11 and light-emitting element 13 takes on a value commensurate with a gate-to-source voltage Vgs of the TFT 11. As a result, the light-emitting element 13 continues to emit light at the brightness commensurate with the current.
The operation adapted to select the scan line WSL and convey the brightness information, which has been given to the data wiring, to the pixel circuit is hereinafter referred to as “writing.”
As described above, the light-emitting element 13 in the pixel circuit 2a shown in FIG. 2 continues to emit light at a constant brightness once the potential Vdata is written. The light-emitting element 13 continues to do so until the potential Vdata is rewritten.
As described above, the pixel circuit 2a controls the current value flowing through the light-emitting element 13 by changing the voltage applied to the gate of the TFT 11 which serves as a drive transistor.
At this time, the p-channel drive transistor has its source connected to the supply potential VCC. As a result, this TFT 11 operates in the saturated region at all times. Therefore, the TFT 11 serves as a constant current source whose current has the value shown in Equation 1 below.(Equation 1)Ids=½*μ(W/L)Cox(Vgs−|Vth|)2  (1)
Here, μ is the carrier mobility, Cox the gate capacitance per unit area, W the gate width, L the gate length, Vgs the gate-to-source voltage of the TFT 11, and Vth the threshold of the TFT 11.
With a simple matrix image display device, each light-emitting element emits light only instantaneously when selected. In contrast, with an active matrix display device, the light-emitting elements continue to emit light even after the writing is complete, as described above. As a result, an active matrix display device can provide high peak brightness and reduced peak current as compared to a simple matrix display device, making this type of display device advantageous particularly when it is used in a large-size, high-definition display.
FIG. 3 is a view illustrating the secular change of the current vs. voltage (I-V) characteristic of the organic EL element. In FIG. 3, the curve shown by a solid line represents the characteristic in the initial state, whereas the curve shown by a dashed line represents the characteristic following a secular change.
The I-V characteristic of the organic EL element generally deteriorates over time as illustrated in FIG. 3.
However, the dual-transistor drive circuit shown in FIG. 2 is driven by a constant current. As a result, a constant current continues to flow through the organic EL element. This keeps the organic EL element free from secular deterioration of emission brightness even in the event of a deterioration of the I-V characteristic thereof.
Incidentally, the pixel circuit 2a in FIG. 2 includes p-channel TFTs. However, if the same circuit 2a includes n-channel TFTs, the existing amorphous silicon (a-Si) process can be used to manufacture TFTs. This provides reduced costs for TFT substrates.
Next, a basic pixel circuit will be described in which p-channel TFTs are replaced by n-channel TFTs.
FIG. 4 is a circuit diagram illustrating a pixel circuit which includes n-channel TFTs in place of p-channel TFTs shown in FIG. 2.
A pixel circuit 2b shown in FIG. 4 includes n-channel TFTs 21 and 22, a capacitor C21, and an organic EL element (OLED) 23 which is a light-emitting element. In FIG. 4, DTL and WSL represent the data wiring and scan line, respectively.
In the pixel circuit 2b, the TFT 21 serves as a drive transistor. The TFT 21 has its drain connected to the supply potential VCC and its source connected to the anode of the EL element 23, thus forming a source follower circuit.
FIG. 5 is a view illustrating an operating point of the TFT 21, which serves as a drive transistor, and the EL element 23 in the initial state. In FIG. 5, a drain-to-source voltage Vds of the TFT 21 is plotted along the horizontal axis, and a drain-to-source current Ids thereof along the vertical axis.
As illustrated in FIG. 5, the source voltage is determined by the operating point of the TFT 21, which serves as a drive transistor, and the EL element 23. This voltage varies depending on the gate voltage.
The TFT 21 is driven in the saturated region. As a result, the current Ids shown in Equation 1 flows through the TFT 21. The current Ids is related to Vgs which is associated with the source voltage at the operating point.